Concerns of possible global ozone depletion (e.g., WMO/UNEP, 1994) have led
to major international programs to monitor and explain the observed ozone
variations in the stratosphere. In response to these, and other long-term
climate concerns, NOAA has established routine monitoring programs using both
ground-based and satellite measurement techniques (OFCM, 1988).

Selected indicators of stratospheric climate are presented in each Summary
from information contributed by NOAA personnel. A Summary for the Northern
Hemisphere is issued each April, and, for the Southern Hemisphere,
each December. These Summaries are available on the World-Wide-Web at the
site: http://www.cpc.ncep.noaa.gov
with location: products/stratosphere/winter_bulletins

ABSTRACT
Ozone measurements during the winter of 1995-1996 indicate that total column
ozone values were substantially lower than values observed during these months
in 1979 and the early 1980's. Over the north polar regions, the Barents Sea,
Greenland, northern Europe and northern Siberia, total ozone for March 1996 was
lower by 20 to 25 percent than during the earlier period. Total ozone has
decreased since 1979 over the Northern Hemisphere mid-latitudes at the rate of
about -4 percent per decade. Little or no significant long-term trend is
observed for the equatorial region. Lower stratosphere winter temperatures over
the north polar region reached record low values. Temperatures observed were
sufficiently low within the polar vortex for chemical destruction of ozone on
polar stratospheric clouds .

I. DATA RESOURCES

The data available and appropriate references are listed below. This
combination of complementary data, from different platforms and sensors,
provides a strong capability to monitor global ozone and temperature.

GROUND-BASED OBSERVATIONS

Parameter

Method

Reference

Total Ozone

Dobson

Komhyr et al., 1986

CMDL, 1990

Ozone Profiles

Balloons

Komhyr et al., 1989

CMDL, 1990

SATELLITE OBSERVATIONS

Parameter

Method

Reference

Total Ozone

NOAA/SBUV/2

Planet et al., 1994

Nimbus-7 SBUV

Mateer et al., 1971

Ozone Profiles

Miller, 1989

Planet et al., 1994

Mateer et al., 1971

Temperature Profiles

NOAA/TOVS

Gelman et al., 1986

We have used the total column ozone data from the NASA Nimbus -7 SBUV
instrument from 1979 through 1988, the NOAA-11 SBUV/2 from January 1989 to
August 1994, and the NOAA-9 SBUV/2 instrument beginning September 1994. Solar
Backscatter Ultra-Violet instruments can only produce data for daylight-viewing
conditions, so no SBUV/2 data are available at polar latitudes during winter
darkness conditions. In addition, increasing data loss of NOAA-11 data, at
sub-Arctic latitudes, was caused by satellite precession over several years and
resulting changes of SBUV/2 viewing to later times of the day.

II. DISCUSSION

Anomalies of zonal mean total column ozone are shown in Figure 1,
as a function of latitude and time, from January 1979 to March 1996. The
monthly mean anomalies (percent difference) are derived relative to each month's
long-term, 1979-1996, average. For the winter of 1995-96, negative total ozone
anomalies of greater than 10 percent are seen for the Northern Hemisphere Arctic
latitudes. Thus in recent years, zonal mean total ozone at high-latitudes,
was substantially lower, by up to 25 percent, than in the earlier years. At
mid-latitudes, the anomaly was slightly positive during 1995-96, in contrast to
the widespread negative anomalies in 1992-93 and 1994-95. Large negative
anomalies in the Northern Hemisphere extra-tropics during 1992-1993 (Gleason et
al., 1993) could be related to the Mt. Pinatubo eruption in mid-1991. Stolarski
et al. (1992), Hollandsworth et al. (1994) and Miller et al. (1994) have
indicated that middle latitude Northern Hemisphere total ozone trends of about
-2 to -4 % per decade are statistically significant, and that little or no
significant trend exists over the equatorial region. For the region 30N-50N
(the basic latitude range of the coterminous United States), the trend, based on
the SBUV - SBUV/2 data sets, and updated from 1979 through March 1996 is about
-4 percent per decade, with a 95 percent confidence estimate of about 2 percent.
In the tropical region, a weak low anomaly is seen in 1995-96, as part of the
quasi-biennial oscillation of total ozone.

The NOAA Climate Monitoring and Diagnostics Laboratory operates a 16-station
global Dobson spectrophotometer network for total ozone trend studies.
Recently, the data have been reanalyzed, and ozone trends re-calculated for the
period 1979 through 1995. Figure 2 shows the corrected total ozone
data for four central U.S. stations. The large annual variation is a result of
ozone transport processes which cause a winter-spring maximum and a summer-fall
minimum at northern midlatitudes. Monthly means for the period 1979-1995 have
been subtracted from each individual monthly mean, and are shown in Figure 3
as a four-station average percent deviation. The resulting trends, for the
1979-1995 period for seven northern hemisphere stations, are given in Table
1.

Table 1. Ozone Trends for the Period 1979-1995

Station

Latitude

Trend

95%

(percent/decade)

confidence

Caribou, ME

46.9°N

-4.00

1.42

Bismarck, ND

46.8°N

-3.30

1.26

Boulder, CO

40.0°N

-3.85

1.22

Wallops Island. VA

37.9°N

-3.67

1.36

Fresno, CA

36.8°N

-3.81

2.74

Nashville, TN

36.3°N

-3.09

1.40

Mauna Loa, HI

19.5°N

-0.53

1.54

Northern hemisphere distributions of ozone and ozone changes are illustrated
in the next two figures. Monthly mean total ozone amounts for March 1996 are
shown in Figure 4. Lowest values are shown over the north polar region
(green, less than 300 DU), extending from Greenland, to northern Europe and
northern Siberia. Total ozone values of less than 250 DU are typical for
tropical values, but are unusual for the polar region. Indeed, at the
beginning of March 1996, extremely low values of total ozone (near 200 DU)
were observed over northern Europe. The March 1996 monthly mean map also shows
a region of high ozone (yellow and red colors), typical for this region and
season, located over middle to high northern latitudes. Figure 5
shows the percent difference in monthly mean total ozone, between the
distribution during March 1996 and the mean for eight March monthly means,
1979-1986. The 1979 to 1986 base period is chosen because these values are
indicative of the early data record. Decreases of 20 to 25 percent (blue)
cover a very large area over the Arctic, from Greenland, over the Baltic Sea,
to northern Europe and northern Siberia. Over western United States, March
1996 values were lower than those for March 1979-1986 by 2 to 4 percent. Small
percent increases are shown over some local areas of the tropics and
mid-latitudes, but these increases are short-term, regional effects, and are not
representative of general, long-term trends of ozone.

Temperatures in the lower stratosphere are closely coupled to ozone through
dynamics and photochemistry. Extremely low temperatures (lower than -78 C)
over the Arctic region in the lower stratosphere are believed to lead to
depletion of ozone. Low temperatures contribute to the presence of polar
stratospheric clouds (PSCs). PSCs enhance the production and lifetime of
reactive chlorine, leading to ozone depletion (WMO/UNEP, 1994).

Daily minimum temperatures over the polar region, 65N to 90N at 50 mb
(approximately 19 km) are shown in Figure 6. We see that for much of
the winter of 1995-96, the daily minimum temperatures were near record low
values, and were sufficiently low (lower than -78 C) for polar stratospheric
clouds to form and allow enhanced ozone depletion. Indeed, during the entire
1995-96 winter, minimum 50 mb temperatures were below the long-term average
minimum temperatures

Temperature anomalies for the 100-50 mb layer derived from radiosonde data
(updated from Angell, 1988) are shown in Figure 7. The winter of
1995-96 had the lowest temperatures ever for the Northern Hemisphere as a whole
and for the north polar region. Figure 8 at 50 mb for three latitude
regions, 65N-90N, 25N-65N, and 25N-25S (updated from Gelman et al., 1986) shows
that temperature anomalies for 1995-96 were near record low values for polar
and equatorial latitudes. However for recent months, near average temperatures
prevailed over middle latitudes.

Aerosol concentration is another important component of stratospheric
variation, and a possible source of ozone depletion (e.g. Hofmann et al.,
1992). Aerosol optical thickness from the NOAA/AVHRR instrument (Long and
Stowe, 1993) showed that stratospheric aerosol concentrations continued to
diminish from the maximum values observed a few weeks after the eruption of
Mount Pinatubo in June 1991. Stratospheric aerosols were at such low levels in
1994, that it was difficult to discern stratospheric aerosols from variations
in tropospheric values. The NOAA 11 AVHRR instrument failed in September 1994.

Recent studies have demonstrated the importance of heterogeneous chemistry,
not only in Antarctica, but also at mid-latitudes. Chlorine-catalyzed ozone
loss processes are enhanced following major volcanic eruptions which inject
sulfur into the stratosphere, and greatly increase the surface area of
stratospheric aerosols. Several eruptions have strongly influenced the
stratospheric aerosol and ozone contents of past decades. Solomon et al.
(1996a) showed that for an atmosphere with pre- anthropogenic (natural) levels
of chlorine, slight ozone increases are expected for mid-latitudes following
volcanic eruptions. Current understanding suggests that the increasing levels
of chlorine caused by human activities interact with volcanic aerosols, and lead
to decreases of stratospheric ozone.

Using a state-of-the-art two-dimensional dynamical-chemical model of the
middle atmosphere, with aerosol variability prescribed from SAGE I, SAGE II and
SAM observations of extinction, Solomon et al. (1996a) showed that it is highly
likely that much of the variability of the northern hemisphere mid-latitude
ozone observed from 1979 through 1994 was induced by the modulation of chlorine-
catalyzed ozone loss by volcanic aerosols. Updating those findings through the
winter of 1995-1996, Solomon et al. (1996b) examined the recovery of the ozone
depletion from the eruption of Mt. Pinatubo. Figure 9 shows the
calculated and observed ozone column changes for 45 N. Both the model and the
observations represent 25-month running means, in order to remove the effects of
the quasi-biennial oscillation. The smoothed time series as shown extends to
the end of 1994, but reflects observations through the end of 1995.

The abrupt onset of the ozone loss in the model, in the early 1980's, is
caused by the effect of the 1982 El Chichon aerosols in the model. During the
period from 1985 to that just before Pinatubo in 1991, aerosols decreased, while
total chlorine increased due to human activities, causing a flattening of the
ozone trend. Following Pinatubo, a much larger ozone loss was shown from
observational data and model calculations. The recent moderate recovery of
northern hemisphere mid-latitude total ozone, as shown by SBUV and SBUV/2
observations, is also remarkably well-simulated. In contrast to the period
after El Chichon, the absence of a continuing large trend in stratospheric total
chlorine after Pinatubo (because of control measures) implies that such a
recovery is expected. This work strongly suggests that aerosols have controlled
much of the observed variability in ozone trends since 1979. Similarly, it is
to be expected that the recovery of stratospheric ozone in the next several
decades will not be monotonic, but will show fluctuations that follow major
volcanic eruptions. We note that Solomon et al. (1996a) also discussed the need
to consider temperature fluctuations together with volcanic aerosol content when
considering ozone losses at higher latitudes.

III. CONCLUDING REMARKS

Observed total ozone values continued to be very low over high latitude
regions of the Northern Hemisphere during the winter of 1995-96. Lower
stratosphere temperatures over the north polar region also reached record low
values. A full explanation of ozone and temperature anomalies must include all
aspects of ozone photochemistry and meteorological dynamics.